1. Field of the Invention
This invention is directed to casing or tubular centralizers with flexible bow springs for use in bore bole drilling operations, and particularly to centralizers that can be radially compressed to fit within a small annular space and which can later expand to center casing within the bore hole.
2. Description of Related Art
Casing is tubular pipe used to line a bore hole that is drilled to recover naturally occurring oil or gas deposits. A joint of casing is typically about 36 to 44 feet in length, and generally has threaded connections at each end to facilitate coupling to adjacent joints of casing to form a casing string. Casing prevents drilled bore holes from washing out or caving in during subsequent drilling and completion operations. Casing strings are cemented into place in the bore hole by circulating cement from the surface down the interior of the casing string, and then displacing the cement into the annular space between the exterior surface of the casing string and the wall of the bore hole. It is important that the casing string be positioned prior to cementing as closely as possible to the center of the bore hole in order to ensure full circumferential cement placement around the casing and to thereby effectively isolate and seal off penetrated geologic formations and to prevent unwanted fluid flow. It is desirable to install casing periodically to maintain the bore hole, and the length of a casing string that can be effectively installed and cemented into place in the bore hole is limited. Progressively smaller diameter casing strings are lowered into the bore hole through the interior of previously installed and cemented casing. Consequently, progressively smaller diameter casing is installed as the depth of the drilled bore hole increases.
It is advantageous to case the bore hole adjacent to producible hydrocarbon zones with casing of a reasonably large diameter to facilitate completion and production operations. This advantage is achievable using bi-centered drill bits. Bi-centered drilling bits are drilling bits that can be used to drill a bore hole of a size larger than the inside diameter of cemented casing through which the drilling bit passes. For example, FIG. 1A illustrates a bore hole having casing 90 with 13xe2x85x9c inch outside diameter and 12xc2xc inch inside diameter. Using conventional drilling bits, the bore hole below the 12xc2xc inch I.D. casing 90 can be drilled only to 12xc2xc inch in diameter 71, and the largest standard size of casing that can be effectively installed and cemented in the bore hole is 9⅝ inch O.D. casing 82. However, using a bi-centered bit allows drilling the bore hole beneath the end of the 12xc2xc inch I.D. casing 90 to 14xc2xe inch diameter 72 as shown in FIG. 1B. This advance in drill bit technology allows 11xe2x85x9e inch O.D. casing 80 to be effectively cemented in place beneath the 12xc2xc inch I.D. casing 90 as an annulus 75 comparable to the annulus between the 9⅝ O.D. inch casing and the 12xc2xc O.D. inch bore hole in FIG. 1A is obtained around the 11xe2x85x9e inch O.D. casing in FIG. 1B.
The bore hole is easier to drill and complete if larger diameter tools can be used. As wells are drilled to deeper depths, and as more high angle and horizontal wells are drilled, the number of discrete sizes of casing used in the casing system increases, and the available annular clearance between adjacent strings of casing necessarily decreases. For example, referring again to FIG. 1B, if a bi-centered bit is used to drill the bore hole below a string of 12xc2xc inch I.D. casing 90 to 14xc2xe inch, and if 11xe2x85x9e inch O.D. casing 80 is lowered through the 12xc2xc inch I.D. casing 90 and into the 14xc2xe inch diameter 72 bore hole, the thickness of the annular clearance 74 between the inside of the (fixed) 12xc2xc inch casing 90 and the outside of the 11xe2x85x9e inch O.D. casing 80 is {fraction (3/16)} inch. The use of bi-centered bits gives rise to the need for a centralizer that substantially radially collapses to fit within the thin {fraction (3/16)} inch annular space 74, and that can later re-deploy to center the 11xe2x85x9e inch O.D. casing 80 in a 14xc2xe inch diameter 72 bore hole. However, as the thickness of the annular clearance 74 becomes very small, it becomes difficult to obtain optimal pre-cement centralization of casing 80 using conventional bow-spring centralizers because they require excessive annular clearance 74 between adjacent casing strings 90 and 80.
Bow-spring centralizers are used to center casing inside a drilled borehole in order to obtain uniform annular placement of cement in the casing/bore hole annulus 75. Bow springs extend radially outwardly from the center bore of the centralizer to provide desirable casing stand-off from the wall 76 of the bore hole. Wide-deployment centralizers are centralizers designed to provide substantial stand-off from a nearby object such as the wall of a bore hole. For example, centering a 11xe2x85x9e inch O.D. casing within a 14xc2xe inch diameter bore hole requires radial stand-off of about 1{fraction (7/16)} inch. By comparison, a limited deployment centralizer may be used to center a 4xc2xd inch O.D. diameter casing within a 5xc2xd inch diameter bore hole, and requires only a radial stand-off of about 0.5 inch. While a limited stand-off centralizer may have bow springs that are sufficiently stiff to resist radially outward collapse upon being pushed into or through a restriction, most wide-deployment centralizers have bow springs that are flexible, and the leading ends of the bow springs must be secured against the casing in order to prevent radially outward collapse of the centralizer ribs. As shown in FIG. 2A and FIG. 2B, bow-spring centralizers are secured to the exterior of the casing and centralizer ribs project radially outwardly therefrom. In order to provide optimal centralization of casing installed at lower depths in the well, it is essential that centralizers be resiliently collapsible to fit within the annular space between the exterior surface of the casing being installed and the interior surface of the larger, installed casing. It is also desirable that the centralizer ribs 42 collapse radially inwardly to achieve a minimum annular clearance, and that the centralizer does not prevent the flow of drilling fluid through the annulus between the smaller and larger casing.
Another factor to be considered in designing a low annular clearance centralizer is the prevention of centralizer rib damage. No wellbore is perfectly vertical and uniform. As casing is lowered into a well, the high strength steel resiliently twists, turns and flexes as it passes through restrictions and bends in the non-linear and non-uniform bore hole. It is important to prevent damage to centralizer ribs by securing the centralizer to the casing in a manner that allows free rotation of the centralizer relative to the casing to which it is secured.
Another factor to be considered in designing low annular clearance centralizers is related to securing the centralizer in place on the exterior of casing that is to be lowered into the well. As the bow springs radially inwardly collapse, the ends of each bow spring must longitudinally separate one from the other. Longitudinal elongation of the centralizer requires that at least one of the collars to which the end of bow springs are secured must remain longitudinally movable relative to the casing on which the centralizer is secured. If the other collar is secured to the casing, then the centralizer will be unidirectional; that is, the centralizer ribs will collapse only when the centralizer passes through a restriction in one direction, and the centralizer ribs will not collapse and pass through the larger casing unless the collar that is secured to the casing is the leading collar to first enter the larger, installed casing. However, a centralizer needs to be bi-directional in order to allow casing to be reciprocated or withdrawn from the bore hole if problems arise during casing installation, and this requires that collars at each end of the bow springs be longitudinally movable relative to the casing.
One attempt to provide a bi-directional centralizer involves the fixed placement of a stop collar longitudinally between sliding end collars secured to the ends of the bow springs, and securing the stop collar to the exterior of the casing to be centralized in the bore hole. This configuration provides the desirable bi-directional function of the centralizer because the centralizer will slide along the casing as the ribs resist collapse and entry into the larger casing until the leading end collar abuts against the stop collar. The ribs will then flatten as they enter the opening or restriction, and the longitudinal elongation of the centralizer slidably displaces the trailing end collar in a direction away from the leading end collar abutted against the stop collar.
The problem with locating a stop collar longitudinally between sliding end collars is that the thickness of the stop collar prevents the centralizer ribs from completely radially inwardly collapsing to lie flat along the length of the casing to which the centralizer is slidably secured. This design causes the stop collar to consume valuable annular space and thereby prevents optimal sizing of subsequent casing.
Attempts have been made to develop low annular clearance centralizers; that is, centralizers that collapse radially inwardly to fit within a thin annulus. U.S. Pat. No. 5,575,333 discloses a bow spring centralizer that integrates the bow springs into a specially manufactured thin-walled tubular member, or sub, that threadably couples at each end to standard joints of casing. These tubular members have very thin-walls of high strength material. A problem with the centralizer disclosed in the ""333 patent is that the centralizer ribs are not freely rotatable about the joint of casing to which the centralizer is secured and are, therefore, subject to breakage and damage as the casing is lowered into the bore hole. Broken ribs may cause the casing to be cemented off-center in the bore hole, thereby greatly increasing the likelihood of fluid flow behind casing, casing failure and loss of productivity of the well. Also, broken centralizer ribs may obstruct the bore hole and prevent installation of casing on completion of the well. Another problem with the centralizer disclosed in the ""333 patent is that the overall mechanical integrity of the resulting non-uniform casing string is compromised by the inclusion of the non-standard, thin-walled subs. It is desirable to use casing that conforms to standards promulgated by the American Petroleum Institute (API), and the necessity of installing thin-walled subs requires frequent interruptions in drilling and completion operations. Another problem with the centralizer disclosed in the ""333 patent is that the use of centralizing subs inhibits the placement of centralizers in close proximity one to the other without specially fabricated subs made to accommodate two centralizers in close proximity or specially fabricated subs of varying lengths.
What is needed is a low annular-clearance bi-directional bow-spring centralizer that can be secured to the exterior surface of a joint of standard casing, radially compressed to its annular configuration, and passed along with the casing through the interior of a slightly larger diameter casing, and later radially outwardly deployed within the lower, uncased bore hole to centralize the casing. What is needed is a centralizer that radially collapses to fit within a very thin annular space, and one that still permits sufficient flow of drilling fluid through the annular space between the smaller and the larger casing to reduce risks of swabbing or surging the well as casing is run into or withdrawn from the bore hole.
One embodiment of the present invention provides a bi-directional low annular clearance bow-spring centralizer having, at each end, a displacement assembly comprising a moving collar and a stop collar, and a longitudinal bore therethrough to accommodate the casing to which the centralizer is secured. Each moving collar has a collet with a radially outwardly flanged portion thereon that is movably received within a circumferential groove or bore within its mating stop collar. A plurality of flexible bow springs, each having a first end and a second end, are secured at each end to a moving collar, and the two moving collars are maintained in a variable spaced-apart relationship by the bow springs. The variance in the longitudinal distance between the two opposing moving collars is determined by the configuration of the bow springs. The longitudinal distance between the moving collars is smallest when the bow springs are in their radially outwardly deployed configuration as shown in FIG. 2A and FIG. 2B, and the longitudinal distance between the moving collars is greatest when the bow springs are radially inwardly collapsed so as to be substantially flattened along the exterior length of the casing to which the centralizer is secured. The position of the flanged portion of each moving collar within the circumferential groove or bore of its mating stop collar is determined by the configuration of the bow springs and by the mechanical interaction between the leading end of centralizer and the larger, cemented and installed casing string through which the centralizer passes.
One embodiment of the present invention provides for each stop collar adapted for having three distinct internal bores of differing diameters: the securing bore, the receiving bore and the reciprocation bore, from smallest to largest. The securing bore is sized for securing the centralizer to the exterior circumferential surface of the casing to be centered in the bore hole using the centralizer. The receiving bore is sized to slidably receive the exterior generally cylindrical surface of the collet of the mating moving collar. The larger diameter reciprocation bore is disposed between the securing bore and the receiving bore, and is adapted to accommodate longitudinal reciprocation of a radially flanged portion on the collet of the mating moving collar.
One embodiment of the present invention provides for each moving collar being longitudinally movable relative to its mating stop collar to accommodate longitudinal displacement of the ends of the bow springs. The range of longitudinal movement of the moving collar within its mating stop collar is limited in one direction by a stop wall in the reciprocation bore and in the other direction by the collapsed and flattened length of the bow springs. Each bow spring is radially outwardly biased towards its deployed configuration, bowed away from the center axis of the casing to which the centralizer is secured. The stop collars are secured to the casing at a distance one from the other such that, when the bow springs are in their fully deployed configuration, the radially flanged portion on the collet of the moving collar is in its extreme inboard position abutted against the stop wall of the reciprocation bore of the stop collar. The bow springs collapse toward the longitudinal center of the centralizer as the bow springs are forced radially inwardly by contact with the surface of a bore hole or with the interior wall of larger casing. In the preferred embodiment, the bow springs flexibly collapse to lie substantially flat along the exterior longitudinal length of the casing to which the centralizer is secured.
In the preferred embodiment, each end of each bow spring is secured to a moving collar, and each moving collar is rotatably and slidably coupled to its mating stop collar, so that each end of each bow spring is longitudinally movable relative to the casing to which the centralizer is secured. The stop collars of the centralizer may be secured to the external surface of the casing using a variety of securing means. The stop collars may be heat shrunk onto the casing, adhered to the casing using epoxies or other adhesives, or secured in place by using welding or using connective pins or set screws. Alternately, stop collars may be made in two or more angular pieces, emplaced together to substantially encircle the casing and secured together to form a collar by welding, adhesives, epoxies, or by using connective pins or set screws.
The present invention may be adapted for centering down hole tools that are lowered into the well using a wireline. Many wireline tools need to be centered in the bore hole for optimal performance, and are lowered into the bore hole through restrictions such as tubulars or bore hole deviations. The present invention provides for centralization of wireline tools while maintaining down hole access by wireline and through instructions.
Optionally, the centralizer of the present invention may be used to position an elongate body within a bore hole at a place other than at the center. For example, the centralizer of the present invention may provide only one radially outwardly extending flexible rib for biasing a tool, such as a survey tool, perforating gun, resistivity through-casing tool or other wire line tool, against the interior longitudinal surface of the casing or bore hole in which the tool is disposed. In this application, the present invention may include a device for decentralizing or biasing a tool, and the minimum annular clearance aspect of the present invention is applicable to this use.
The present invention provides a bi-directional, freely rotatable and a radially collapsible centralizer for centralizing a joint of casing to which the centralizer is secured when the casing is lowered into a bore hole. The present invention provides a centralizer that is freely rotatable and substantially radially collapsible to fit within a very thin annular space between two generally concentric casing strings. The present invention provides a centralizer in which each bow spring is substantially collapsible to lie in a substantially flattened configuration along the longitudinal length of the exterior surface of a joint of casing to which the centralizer is secured. The present invention provides a centralizer with two spaced apart moving collars, each moving collar rotatably and slidably coupled to a mating stop collar that is adapted for securing the centralizer to the exterior surface of a joint of casing, and having a plurality of radially outwardly biased bow springs extending between the moving collars and secured at each end to a moving collar.